The present invention relates to the art of diagnostic imaging. The present invention finds particular application in conjunction with nuclear or gamma cameras and will be described with particular reference thereto. The present invention will also find application in other imaging systems such as digital x-ray computed tomography.
In single photon emission computed tomography (SPECT), a radioisotope is injected into the body of a patient. The radioisotope preferably travels to an organ of interest whose image is to be produced. The patient is placed in an examination region of the SPECT system surrounded by large area planar radiation detectors. Radiation emitted from the patient is detected by the radiation detectors. The detectors have a collimator to limit the detector to seeing radiation from a selected trajectory, e.g., normal to its plane. A transmission radiation source is disposed across the patient 180.degree. from one of the detectors. The emission and transmission radiation sources have sufficiently different energies that they are readily differentiated.
Typically, the detector includes a scintillation crystal that is viewed by an array of photomultiplier tubes. The relative outputs of the photomultiplier tubes are processed and corrected, as is conventional in the art, to generate an output signal indicative of (1) an (i,j) coordinate on the detector head at which each radiation event is received, and (2) an energy of each event. The energy is used to differentiate between emission and transmission radiation and between multiple emission radiation sources. A two-dimensional projection image representation is defined by the number of radiation events received at each coordinate.
In tomographic imaging, the detector is rotated in equal increments or in a continuous orbit around the patient. Each projection image is digitized as a two-dimensional array of pixel values or views. These views collected from the multiplicity of projection angles are reconstructed to produce voxel values of a three-dimensional or volumetric image representation. Often, the volumetric image is treated as a series of slice images with each slice being perpendicular to an axis of rotation.
In order to minimize the radiation dosage to which the patient is exposed, the injected radioisotope is of relatively low radioactivity. As a consequence, each view requires a significant amount of time such as, for example, about 40 seconds, to produce. If a total of 64 views on a 360.degree. arc is desired, angularly spaced by about 5.6.degree., then the entire imaging process typically takes about 40 minutes to complete. Blurring or distortion of the resultant image can take place when the body or organs within the body move. Typically, each voxel is about one-half centimeter cubed. Keeping a human body still to within one-half centimeter for 40 minutes is difficult, if not impossible. Thus, body motion and resultant image degradation are common problems.